Longitudinal static stability

[hitech]

For many years it has driven me nuts how the standard pilot books describe pitch stability and CL/CG setups.
Like This which is very useful for a basic understanding.



Full page http://www.dauntless-soft.com/products/freebies/library/books/flt/chapter3/stability.htm

The  diagram above would tend to make most people think plane would pitch down when loosing its horizontal tail planes.

But most fighter planes are set up like the following diagram, as are most plane when set up for fastest cruise.
Because you want both surfaces (wing and H stab) to be lifting, if the tail is pushing down the wing must push up the same amount to have static flight. And producing lift in either direction causes drag. In one case Wing + HStab = weight of plane. in the other (Wing = weight of plane + HSTAB lift) which creates more drag.



The entire page http://en.wikipedia.org/wiki/Longitudinal_static_stability

The misconception comes from the definition of what is Center of lift. Is it the entire lifting surface of the wing and the tail plane, or just the wing.

So at last someone as posted a correct diagram and from and engineering perspective of pitch stability.

HiTech

[earl1937]

For many years it has driven me nuts how the standard pilot books describe pitch stability and CL/CG setups.
Like This which is very useful for a basic understanding.

(Image removed from quote.)

Full page http://www.dauntless-soft.com/products/freebies/library/books/flt/chapter3/stability.htm

The  diagram above would tend to make most people think plane would pitch down when loosing its horizontal tail planes.

But most fighter planes are set up like the following diagram, as are most plane when set up for fastest cruise.
Because you want both surfaces (wing and H stab) to be lifting, if the tail is pushing down the wing must push up the same amount to have static flight. And producing lift in either direction causes drag. In one case Wing + HStab = weight of plane. in the other (Wing = weight of plane + HSTAB lift) which creates more drag.

(Image removed from quote.)

The entire page http://en.wikipedia.org/wiki/Longitudinal_static_stability

The misconception comes from the definition of what is Center of lift. Is it the entire lifting surface of the wing and the tail plane, or just the wing.

So at last someone as posted a correct diagram and from and engineering perspective of pitch stability.

HiTech

Good post and some helpful info, but in the 7 years that I was a full time flight instructor, most all students just needed to know that the 4 forces, thrust, drag, lift and weight were all they needed to know to understand how the aircraft flew! The "flying wing" in the late 40's and early 50's proved you didn't have to have a horizonal stabilizer to fly correctly, you just had to make certain control adjustments on the wing to control the flight of the aircraft. I doubt you could get any of the WW2 era aircraft off the ground without a horizontal stabilizer!

[save]

Maybe except the Ho 229 / GOTHA 229 - Luftwaffe jet fighter

http://www.youtube.com/watch?v=RtNr9mb6CZI

[FTJR]

Thanks for the diagram HiTech, it is easier to visualise as to why load our planes with rear CoG.

Cheers

[icepac]

Much of this was used in Rutan's comparisons of standard plane construction vs his canard creations.

[Ack-Ack]

Maybe except the Ho 229 / GOTHA 229 - Luftwaffe jet fighter

http://www.youtube.com/watch?v=RtNr9mb6CZI

And the Northrop N-9M or the N-1M.

[pembquist]

Does the cg range of a trainer ever go aft of the aerodynamic center of the main wing?

I'm a little confused. I had never thought about what should happen when you lose the horizontal stab but now that You mention it I would have thought that it would be aoa/speed/flap dependent.  In ah if you lose your hor stab while landing what happens? Shouldn't you pitch down? Like a tailplane stall from ice.

[Mister Fork]

Hitech - how does static stability then relate to longitudinal? I know most WWII fighters were designed with a 'neutral' static stability - meaning if you left the inputs alone, it continues along that exact course where most modern aicraft have a positive stability - they are designed to remain upright and with an inclination for a slight climb.

[pembquist]

Mister Fork these are the three kinds of static pitch stability: Neutral pitch stability would mean that if the airplane were trimmed for level flight for instance and you pushed/pulled/or were buffeted changing the airplanes pitch attitude the airplane would remain in the new attitude without control inputs. Positive pitch stability means that without control inputs the same aircraft would tend to return to the attitude and speed it had before the change.  Negative stability would mean that without control inputs the attitude would keep diverging from the original.  For instance if an aircraft with negative stability in level flight encounters a little turbulence leading to a slight nose down attitude, unless the pilot pulls back on the stick the slight nose down attitude will steepen into a vertical dive.

[pembquist]

Mister Fork I failed the reading comprehension test. Longitudinal just refers to stability in the pitch attitude as opposed to roll or yaw (lateral and directional).

[MK-84]

^Speaking of failing at comprehension...

I am fairly certain I understand the premise of his post, and how the diagram relates. but I do not suppose someone could literally dumb this down by replacing the diagrams letters with explanations?

I get what Hitech is stating, I really do, but apparently my brain can not effectively relate to and make sense of the diagram vs his description.

[pembquist]

MK4: If you understand the first diagram than its easy to understand the second. Relabeling the second diagram with terms from the first Lw is lift Lt is tail force W is weight. The benchmark target circle thing is the center of gravity on both diagrams Xg is the distance of the center of gravity from the Aerodynamic Center of the main wing. The clockwise circle with an arrow head is the Pitching Moment of the main wing. The difference between the diagrams is that the first makes it hard to see how you could have lift in a positive (up) direction without the plane going into a dive.  The second diagram makes it easy to see how both the main wing and the tail could be generating postive lift while maintaining level flight. The first diagram shows a seesaw with the main wing being the pivot point while the second shows something more like two guys (the main wing and the tail) carrying a board between them with a weight resting on the board somewhere in between.

The thing that is confusing is that the arrows and distances don't have to be positive numbers. You could say that the first diagram is the same as the second diagram when Xg is negative Lw is positive and Lt is negative. The important thing to understand is the concept of moment. A moment could be described as what a nut experiences when a wrench is used on it. A stable state is when the moment forces cancel each other out. Imagine your trying to loosen a nut with 2 wrenchs, think about what you have to do with two different length wrenchs.  Now just imagine that the benchmark circle thing (CG) on the diagram is the bolt and one wrench is the fuselage going to the tail and the other is the fuselage going to the main wing.

Since you don't get something for nothing lift causes drag.  In the first diagram both the wing and the stabilizer are generating lift just in opposite directions.  The ideal would be if both surfaces could generate lift in the positive direction because than there would be no "wasted" lift.  Using the seesaw/two guys and a board analogy you can see how you could do this.  Imagine the wrench analogy again, if the wrenchs are at 180 degrees you would have to put your foot on the bolt or else it would rise (like an airplane) if they are less than 90 degrees the bolt stays still (airplane can't take off.)

[Motherland]

I'm guessing alpha is the angle of attack, I sub t is the moment of inertia of the tail? What is n?

[FLS]

N is the elevator deflection.

[MK-84]

MK4: If you understand the first diagram than its easy to understand the second. Relabeling the second diagram with terms from the first Lw is lift Lt is tail force W is weight. The benchmark target circle thing is the center of gravity on both diagrams Xg is the distance of the center of gravity from the Aerodynamic Center of the main wing. The clockwise circle with an arrow head is the Pitching Moment of the main wing. The difference between the diagrams is that the first makes it hard to see how you could have lift in a positive (up) direction without the plane going into a dive.  The second diagram makes it easy to see how both the main wing and the tail could be generating postive lift while maintaining level flight. The first diagram shows a seesaw with the main wing being the pivot point while the second shows something more like two guys (the main wing and the tail) carrying a board between them with a weight resting on the board somewhere in between.

The thing that is confusing is that the arrows and distances don't have to be positive numbers. You could say that the first diagram is the same as the second diagram when Xg is negative Lw is positive and Lt is negative. The important thing to understand is the concept of moment. A moment could be described as what a nut experiences when a wrench is used on it. A stable state is when the moment forces cancel each other out. Imagine your trying to loosen a nut with 2 wrenchs, think about what you have to do with two different length wrenchs.  Now just imagine that the benchmark circle thing (CG) on the diagram is the bolt and one wrench is the fuselage going to the tail and the other is the fuselage going to the main wing.

Since you don't get something for nothing lift causes drag.  In the first diagram both the wing and the stabilizer are generating lift just in opposite directions.  The ideal would be if both surfaces could generate lift in the positive direction because than there would be no "wasted" lift.  Using the seesaw/two guys and a board analogy you can see how you could do this.  Imagine the wrench analogy again, if the wrenchs are at 180 degrees you would have to put your foot on the bolt or else it would rise (like an airplane) if they are less than 90 degrees the bolt stays still (airplane can't take off.)

That was perfect, thank you!!